Tame troublesome tapping operations.
Tapping is by nature a difficult metal removal operation. A tap is unusual in that it is required to perform a more severe task than most other types of cutting tools. In other metalcutting operations, the feedrate can be varied depending on desired dimensional accuracy, surface finish, and tool life. A tap, however, must cut at a rate that coincides exactly with the thread lead.
A 1/4 -20 tap, for example, has 20 threads per inch of linear length. This translates into a 0.050" pitch or lead, which represents the exact amount that the tap must advance into the hole in one revolution.
Compounding the problem is the fact that the tap is in effect a single-point cutting tool. After the first few chamfer teeth, all cutting is performed by the first full tooth, which gives the thread form its final finished shape and size. All the succeeding teeth beyond the first full tooth do nothing but follow in the pathway formed by the first full tooth.
Making good threads can be difficult because the root causes of tapping problems are a mystery to many, believes Al Bolden of Grosse Pointe Woods, MI, consultants Incom Corp. Getting to the bottom of tapping problems consumed a large part of the life of TM Smith, Mr Bolden's mentor and the founder of tapholder manufacturer TM Smith Tool International Corp, Mt Clemens, MI. Mr Bolden now uses the principles developed by TM Smith to diagnose tapping problems on a consulting basis. This article is based on information from TM Smith, interviews with Mr Bolden and current TM Smith Tool president Fred Smith, and on Mr Bolden's videotape, The Mysteries of Tapping, which is available through Incom.
Part of the problem lies in how thread quality is measured, according to Mr Bolden. Commonly used go/no-go gages provide little or no information about the causes of thread size problems. "People think go/no-go plug gages measure diameter," he says. "That's not true. They measure tooth space width."
Threads that gage oversize are the most common problem encountered in tapping today. Although there are cases where an oversize threaded hole is actually too large peripherally, the real problem usually is that the tooth space is too wide and permits the no-go gage to enter the hole. The tooth space is too wide because the tap has either been "pushed" to a feedrate exceeding its own lead or has been "retarded," prevented from keeping up with its own lead. The same kind of condition can result from "pulling" during tap retraction.
If the tap is being pushed, the top flank angle of the thread form in the workpiece will be shaved. If it is being retarded in its infeed, or being pulled during retraction, the bottom flank angle will be shaved. Either way, the effect is a wide tooth space, which translates into an oversize pitch diameter. In such cases, thread root and crest (minor and major) diameters are usually only nominally affected. Thus the thread's peripheral size is within tolerance, and the actual error is in the wide tooth space.
In an undersize threaded hole--one in which the go gage will not enter--the opposite is true: the problem isn't that the peripheral size of the threaded hole is too small; rather, the tooth space has been "mashed" and is too narrow. Generally, this condition is limited to the top one to three threads.
The mashing results when the combined weight of the tap, tap holder and machine spindle push back against the tap as it leaves the hole. When only a small amount of contact area remains between the tap and the workpiece, this weight will push the tap back into the workpiece and the first one to three threads will be bent or mashed downward.
Operators can check for mashed threads by removing the tap from the machine spindle and re-shaving the first few threads by hand. This will usually correct the bent or mashed condition, and provide proof of the root cause of the problem.
CNC tapping troubles
Push, pull, and retard problems can occur on both non-leadscrew and leadscrew machines. Tapping on a CNC machine without canned cycles for synchronous tapping operations has its own set of complications, but the symptoms of threading problems--oversize tooth forms or mashed threads--remain the same.
It may be difficult, for example, to program the feedrate to coincide exactly to the lead of the tap. To give the tap the greatest freedom to produce an accurate thread form, it is usually best to program the machine spindle to feed slightly slower than the tap lead. This permits the tap to advance according to its own lead, with the spindle giving only a minimal assist. A tension-type tap holder with a ball bearing tension drive is recommended for this type of operation. Another important factor is programming the rapid return cycle of the machine spindle. Often, this movement is programmed to coincide closely with the disengagement of the tap with the hole. Since the tap holder is a tension-type, it will pull out or extend itself during the infeed tapping cycle and remain in the extended mode during the reverse cycle, finally springing back when the tap is completely disengaged from the workpiece.
However, if the tap starts cutting slightly sooner on one piece than on another due to tolerance stack-up on the workpiece, variation in pre-tap hole size, or other factors, the tap will feed deeper into the hole and will require more time to back out. If rapid spindle retraction is programmed too close to the mathematical disengagement point, the tap could still be engaged when the rapid return cycle starts. This can cause a slight "pull-out" of the tap at the end of each cycle, resulting in loss of depth control.
Looking for clues
Evidence of the pushing, pulling, and retarding that often cause size problems in threaded holes can be found by close examination of the taps themselves. To use "witness marks" on taps to diagnose thread problems, the tap must be used at least long enough to have established wear pattern marks on the teeth. The longer a tap has been used, the more prevalent the marks will be and the easier they are to detect. Taps used only a short time or those that lack a surface treatment on which wear marks are more easily seen may require a magnifying glass for examination.
The tap should be held tipped downward about 30 deg in a position that lets you see both the end face and the faces of the tooth Clanks. Examine the flank faces closely for evidence of rubbing or scuffing. A close look with a magnifying glass and a comparison of the first five to eight teeth with the last two or three teeth of the tap should show a marked difference in the wear pattern. If the tap produced good threads and had adequate life, wear marks will be uniform and minimal. Turn the tap around and examine the back faces of the flanks as you did the front. If the tap has operated satisfactorily, the wear marks should be uniform and minimal on both the front and back flank faces.
If the marks are more severe on the front flank angles, the tap has been pushing. If they are predominant on the back flanks, it has been retarded or pulled on the reverse cycle. If you observe either of these wear patterns, you've probably experienced size control problems with the tap. If the scuff marks are severe and of equal consistency on both flank faces, the tap probably has experienced the dual adversity of both push and pull.
When you observe any of these conditions, the crest of the tooth form on the tap could also be severely scuffed. This is caused when the root of the thread in the workpiece is elongated or widened by the crest of the tap. Thus, another possible sign of adverse pressures on the tap can manifest itself in the scuffed crest form of the tap.
Another common problem affecting thread size--one that often goes undetected--is attempting to cut with the flat end face of the tap. Since the end face of the tap cannot cut, it will by sheer force push the metal ahead of itself. This will create scuff marks at the outer corners on the end face of the tap. If the tap has had a nitride, oxide, or plating surface treatment, the scuff marks will be more readily visible than an uncoated or untreated tap. Sometimes, the chip flow along the face of the flutes will also produce a visible scuff or wear pattern mark. If this flow mark or wear pattern appears to start below the point diameter of the tap, it is a sure sign that the workpiece material has been pushed or extruded ahead of the tap and that the tap is attempting to cut on its flat end face.
If you note any of these signs, a spiral point (not spiral flute) tap may help. If a spiral point tap is not available and a "quick fix" is required, a chamfer similar to those on the ends of reamer teeth can be ground on the corners of the end teeth of the existing tap. These chamfers should be relieved or backed off to a sharp edge, and need not be precision ground.
The chamfers will let the tap cut away (ream) the extruded material and will relieve the retarding pressures against the tap in-feed. The chamfers should be large enough to extend below the extruded diameter, which can be determined by measuring the scuff mark diameter across the end face of the tap. Improving inspection of internal threads
The fact that go/no-go plug gages really don't tell you a lot about the real cause of thread size problems is only one of their drawbacks. Screwing the go gage, with its fairly long length of engagement, into and out of the part is a time-consuming process. The result: only a handful of parts out of an entire production run are usually checked.
Solving the time constraint problem--and providing some insight into the actual condition of threads in the hole--is a non-contact inspection method that uses a capacitive imaging sensor to inspect internal threads for small flaws. Developed at the General Motors Technical Center, the signature analysis technology was licensed to Assurance Technologies Inc (ATI), Garner, NC. ATI's SigMA Thread Inspector system can provide high-speed, 100% inspection of threaded holes for critical applications, according to ATI president Keith A Morris. "Flaws such as small chips, burrs, partially missing threads, incorrect diameter, and short threads are detectable," he says. Pitch measurement can provide evidence of the shaved thread flank faces that cause plug gages to indicate an oversize condition.
The sensor is inserted into the hole being inspected by a hydraulic or electric actuator. The sensor scans the entire length of the thread with a 360-deg field of view, and the resulting signature is compared to that of previously tested "good" threads. Deviations from the "good" signature indicate problems. Sensors are available in a range of sizes and configurations, including models for inspection of external threads as well.
Automakers and their suppliers have taken an interest in the thread inspection system. An example application is a turnkey installation at a Delco plant that inspects and sorts brake hose fittings at a rate of 125 parts per hour.
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|Title Annotation:||includes related article|
|Author:||Destefani, James D.|
|Publication:||Tooling & Production|
|Date:||Feb 1, 1994|
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